Method of modifying a surface of a workpiece

ABSTRACT

A method of modifying a surface of a workpiece comprises providing a system comprising a sealed mixing vessel having an interior chamber wherein the workpiece and working bodies are contained. The sealed mixing vessel is then uniaxially vibrated at a frequency between 15 hertz and 1 kilohertz, and at a vibrational amplitude between about 0.2 cm and 3 cm such that the working bodies impact the surface of the workpiece. The method is useful for shot peening and abrasive finishing the workpiece.

TECHNICAL FIELD

The present disclosure broadly relates to processes for modifying asurface of a workpiece.

BACKGROUND

Methods of modifying a surface of a workpiece include, for example,methods of finishing the surface of the workpiece and methods ofhardening the surface of the workpiece.

In the case of molded parts (e.g., especially cast metal parts), it iscommon practice to subject the workpiece to abrasive post-processing toremove burs, mold lines, and otherwise smooth the surface of theworkpiece. Examples of such processes include vibrating and/or blastingwith abrasive media propelled by high velocity gas (e.g., nut shells,ceramic particles, steel balls, or sand). In these processes, unwantedraised surface features are reduced over time.

Shot peening is similar to sandblasting, except that it operates by themechanism of plasticity rather than abrasion: each particle functions asa ball-peen hammer. In practice, this means that less material isremoved by the process, and less dust created.

Shot peening (i.e., peening with shot particles, hereinafter “shot”) isa cold working process used to produce a compressive residual stresslayer and modify mechanical properties of metals and composites. Itentails impacting a metallic surface with shot (i.e., round particlestypically made of, for example, metal, glass, or ceramic) withsufficient force sufficient to create plastic deformation. In machining,shot peening is used to strengthen and relieve stress in components likesteel automobile crankshafts and connecting rods. In architecture itprovides a muted finish to metal. Typically, in shot peening a stream ofshot is directed toward a workpiece.

Both abrasive finishing and shot peening can be manual, time-consumingprocesses that can last for hours or days.

SUMMARY

There remains a need for faster and improved methods of modifying thesurface of a workpiece that involve impact by particles such as abrasiveblasting and shot peening. Advantageously, the present disclosureprovides rapid abrasive blasting and shot peening methods that areenergy efficient and easy to carry out (e.g., no shot recycling ormanual directing of particle streams necessary).

Accordingly, in one aspect, the present disclosure provides a method ofmodifying a surface of a workpiece, the method comprising:

providing a system comprising a sealed mixing vessel having an interiorchamber containing the workpiece and working bodies;

uniaxially vibrating the sealed mixing vessel at a frequency between 15hertz and 1 kilohertz, and at a vibrational amplitude between about 0.2cm and 3 cm such that the working bodies impact the surface of theworkpiece.

Features and advantages of the present disclosure will be furtherunderstood upon consideration of the detailed description as well as theappended claims.

DETAILED DESCRIPTION

Methods according to the present disclosure may be carried out using avibratory system that includes a sealed mixing vessel having aninterior, processing, chamber. The system may further include anactuator (e.g., a mechanical actuator) capable of vibrating the sealedmixing vessel. Preferably, a control module controls the actuator suchthat the sealed mixing vessel vibrates under resonant or near-resonantconditions (e.g., resonant acoustic conditions) throughout the surfacemodification process. Use of vibrationally resonant conditions ensureshigh efficiency use of the supplied energy.

Commercially available mixing devices capable of accomplishing the aboveare marketed by Resodyn Acoustic Mixers, Butte, Mont. Laboratory-scaledevices include LabRAM I and LabRAM II controlled batch mixers. Largescale devices are marketed under the trade designations OmniRAM, RAMS,and RAM 55. These devices typically operate at resonant vibrationalfrequencies of from 20 to up to <1 kHz, preferably 40 to 100 hertz, morepreferably 40 to 80 hertz, and more preferably 55-65 hertz, althoughthis is not a requirement. The vibrating mixers are also characterizedby actuator displacements that are on the order of 0.5 inch (1.3 cm),that may be accompanied by an acceleration g-force, where g=9.8 m/s², ofat least 20-g, 30-g, 40-g, 50-g, or even at least 60-g, although this isnot a requirement. Further details concerning suitable resonant acousticmixers can be found, for example, in U.S. Pat. No. 7,188,993 (Howe etal.) and U.S. Pat. No. 9,808,778 (Farrar et al.).

In practice, the working bodies and the workpiece(s) are disposed withinthe interior chamber. The workpiece may be loose within the interiorchamber or fixed in a given position relative to the sealed mixingvessel (e.g., mounted to a wall of the sealed mixing vessel. The latterconfiguration may be desirable in instances where selective modificationof a portion of the workpiece surface is desired. The latterconfiguration may also be desirable if the workpiece has a large massand/or is delicate, so that collisions between the workpiece and thevessel walls are prevented.

Advantageously, the working bodies ricochet off the sides and top of thesealed mixing vessel during vibration such that the workpiece isbombarded from all angles.

On a volume basis, the working bodies may collectively constitute up to20, 30, 40, 50. 60, 70, or 80 percent of the volume of the interiorchamber, for example. However, in typical use the working bodies maycollectively constitute from 5 to 35 percent of the volume of theinterior chamber, although lesser and greater amounts may also be used.

Useful working bodies may include abrasive bodies and peening bodies.

The abrasive bodies are typically irregular so that sharp-edgedparticles can cut away brittle surface deposits; however, this is not arequirement. Abrasive bodies useful for the methods of the presentdisclosure may include any abrasive bodies that are useful for abrasiveblasting (commonly termed “sandblasting”) or vibratory tumbling. Thereare several variants of the process, using various media; some arehighly abrasive, whereas others are milder. Exemplary materials for theabrasive bodies may include sand, copper slag, nickel slag, coal slag,glass beads, plastic abrasive, crushed glass, silica, steel spheres,steel grit, stainless steel spheres, cut steel wire, ground-up plasticstock, walnut shells, corncobs, aluminum oxide (which includes brownaluminum oxide, heat treated aluminum oxide, and white aluminum oxide),co-fused alumina-zirconia, ceramic aluminum oxide, green siliconcarbide, black silicon carbide, chromia, zirconia, flint, cubic boronnitride, boron carbide, diamond, garnet, sintered alpha-alumina-basedceramic as described, for example, by U.S. Pat. No. 4,314,827(Leitheiser et al.) and in U. S. Pat. Nos. 4,770,671 and 4,881,951 (bothto Monroe et al.). Preferentially, the abrasive bodies are aggregates ofthe aforementioned abrasive particles, bound by polymers, ceramics ormetals, for example.

Usually, the abrasive bodies range in diameter from 0.01 millimeter (mm)to as large as 5 mm, preferably from 0.1 mm to 5 mm; however, this isnot a requirement. In some embodiments, the abrasive bodies may be sizedaccording to an abrasives industry specified nominal grade.

Abrasive particles graded according to abrasive industry acceptedgrading standards specify the particle size distribution for eachnominal grade within numerical limits. Such industry accepted gradingstandards (i.e., abrasives industry specified nominal grade) includethose known as the American National Standards Institute, Inc. (ANSI)standards, Federation of European Producers of Abrasive Products (FEPA)standards, and Japanese Industrial Standard (JIS) standards.

ANSI grade designations (i.e., specified nominal grades) may include:ANSI 4, ANSI 6, ANSI 8, ANSI 16, ANSI 24, ANSI 36, ANSI 40, ANSI 50,ANSI 60, ANSI 80, ANSI 100, ANSI 120, ANSI 150, ANSI 180, ANSI 220, ANSI240, ANSI 280, ANSI 320, ANSI 360, ANSI 400, and ANSI 600. FEPA gradedesignations include P8, P12, P16, P24, P36, P40, P50, P60, P80, P100,P120, P150, P180, P220, P320, P400, P500, P600, P800, P1000, and P1200.JIS grade designations include JIS8, JIS12, JIS16, JIS 24, JIS 36, JIS46, JIS 54, JIS 60, JIS 80, JIS 100, JIS 150, JIS 180, JIS 220, JIS 240,JIS 280, JIS 320, JIS 360, JIS 400, JIS 600, JIS 800, JIS 1000, JIS1500, JIS 2500, JIS 4000, JIS 6000, JIS 8000, JIS 10000, JIS 20000, andJIS 30000.

Alternatively, abrasive bodies can be graded to a nominal screened gradeusing U.S.A. Standard Test Sieves conforming to ASTM E-11 “StandardSpecification for Wire Cloth and Sieves for Testing Purposes.” ASTM E-11proscribes the requirements for the design and construction of testingsieves using a medium of woven wire cloth mounted in a frame for theclassification of materials according to a designated particle size. Atypical designation may be represented as −18+20 meaning that theabrasive particles through a test sieve meeting ASTM E-11 specificationsfor the number 18 sieve and are retained on a test sieve meeting ASTME-11 specifications for the number 20 sieve. In one embodiment, theabrasive bodies have a particle size such that most of the particlespass through an 18 mesh test sieve and can be retained on a 20, 25, 30,35, 40, 45, or 50 mesh test sieve. In various embodiments of thedisclosure, the abrasive bodies can have a nominal screened gradecomprising: −18+20, −20+25, −25+30, −30+35, −35+40, −40+45, −45+50,−50+60, −60+70, −70+80, −80+100, −100+120, −120+140, −140+170, −170+200,−200+230, −230+270, −270+325, −325+400, −400+450, −450+500, or −500+635.

Optionally the sealed mixing vessel may contain a fluid such as, forexample, water. The fluid may contain optional additives such as, forexample, surfactant, defoamer, or in the case of abrasive bodies anetchant (e.g., an alkali metal hydroxide).

Useful peening bodies may include any bodies known for use in shotpeening. Examples include: spherical metal shot (e.g., cast steel, ironsteel, stainless steel, tungsten, molybdenum, tungsten, titanium,tantalum, cobalt-chrome, or cobalt), spherical ceramic/cermet beads(e.g., zirconia, alumina, silicon carbide, or tungsten carbide/cobalt),spherical glass beads, and conditioned (rounded) cut wire (e.g.,conditioned cut steel wire). Conditioned cut wire shot may be preferredin some applications, because maintains its roundness as it is degraded,unlike cast shot which tends to break up into sharp pieces that candamage the workpiece. Conditioned cut wire shot can last five timeslonger than cast shot. Mixtures of two or more working bodycompositions, shapes, and/or sizes may be used. Usually, the peeningbodies range in diameter from 0.1 millimeter (mm) to as large as 3.2 mm,preferably from 0.7 to 1.2 mm; however, this is not a requirement.

When peening a surface finished workpiece (e.g., deburred and/orsmoothed), any peening bodies useful for shot peening may be used inpractice of the present disclosure. Exemplary useful peening mediaincludes rounded metallic (e.g., cast steel, stainless steel,molybdenum, tungsten, titanium, tantalum, cobalt-chrome, or cobalt)particles and conditioned cut wire versions thereof, glass (e.g., glassbeads), ceramic particles (e.g., tungsten carbide, silicon carbide,titanium carbide, corundum, and Zirshot ceramic media (60-70% ZrO₂,28-33% SiO₂, <10% Al₂O₃ marketed by SEPR Saint-Gobain ZirPro, Le PontetCedex, France), and combinations thereof.

Peening may be beneficially practiced on metallic (e.g., includingaluminum, steel, steel forgings and machine parts) workpieces. Theeffect of peening is a surface phenomenon that typically does not exceedseveral hundred microns in depth, so it is typically only necessary thatthe surface of the workpiece be metallic in order to achieve a benefit.However, in many instances the entire workpiece may be metallic.

Methods according to the present disclosure may be especially beneficialfor workpieces, fabricated by powder jet or laser sintering additivemanufacturing (3D printing) methods, since the working bodies and theworkpiece may be free to move within the sealed chamber, the workingbodies (if sufficiently small) can penetrate into interior passages thatare accessible from the surface of the workpiece, and which may not beeasily accessible using other methods.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in theExamples and the rest of the specification are by weight.

The system used for all examples described below was a LabRAM ResonantAcoustic mixer from Resodyn Corporation, Butte, Mont. The machine, whichwas equipped with a sealed mixing vessel, was run at 100% intensity inthe auto frequency mode. Roughness measurements: Ra, were measured usinga MarSurf PS 10 stylus profilometer and Sa roughness measurements wererecorded using a MikroCAD surface metrology system.

Example 1

This example demonstrates abrading aluminum alloy with loose abrasivegrain.

The workpiece was a machined aluminum alloy (Grade BS EN 755 6082-T6),16 mm×3 mm×50 mm cuboid. The surface was scratched by hand with P36coated abrasive to produce an initial surface roughness R_(a) of 6.2microns. The workpiece part was placed in a polypropylene straight-sidedsealed cylindrical container with 102 mm internal height and 52 mminternal diameter. P80 semi-friable fused aluminum oxide BRFPL (175 g,Imerys, Paris, France) was placed in the container along with theworkpiece. The LabRAM was run at 100% intensity in the auto frequencymode for 30 mins. Afterward, the surface roughness R_(a) of theworkpiece was 4.1 microns. The mass loss of the workpiece during thistime period of processing was 0.036 g.

Example 2

This example demonstrates abrading aluminum alloy with loose abrasivegrain and a chemical etchant.

The workpiece was a machined aluminum alloy (Grade BS EN 755 6082-T6),16 mm×3 mm×50 mm cuboid. The workpiece was scratched by hand with P36coated abrasive to produce an initial surface roughness R_(a) of 7.1microns. The workpiece was placed in a polypropylene straight-sidedsealed cylindrical container with 102 mm internal height and 52 mminternal diameter. P80 semi-friable fused aluminum oxide BRFPL (175 g,Imerys) was placed in the container along with the part. 1 M potassiumhydroxide solution (75 ml) was added to the container. The LabRAM wasrun at 100% intensity in the auto frequency mode for 30 mins. Afterward,the roughness R_(a) of the workpiece after 30 mins of processing was 5.1microns. The mass loss of the workpiece during this time period ofprocessing was 0.094 g.

Example 3

This example demonstrates abrading aluminum alloy with abrasiveagglomerates.

The workpiece was a machined aluminum alloy (Grade BS EN 755 6082-T6),16 mm×3 mm×50 mm cuboid. The workpiece was scratched by hand with P36coated abrasive to produce an initial surface roughness R_(a) of 5.1microns. The workpiece was placed in a polypropylene sealed cylindricalcontainer with 55 mm internal height and 80 mm internal diameter.Premium Ceramic Fast Cutting Triangles (75 g, 2 mm×2 mm, KramerIndustries, Piscataway, N.J.) were placed in the container along withthe workpiece and 50 g of water. The LabRAM was run at 100% intensity inthe auto frequency mode for 30 mins. Afterward, the surface roughnessR_(a) of the workpiece was 2.6 microns. The mass loss of the workpieceduring this time period of processing was 0.024 g.

Example 4

This example demonstrates abrading additively manufactured aluminumalloy with abrasive agglomerates.

The workpiece was an additively manufactured aluminum alloy (AlSi 10 Mg)20 mm diameter tube with 2 mm walls. The part was printed by DirectMetal Laser Sintering (DMLS). The initial roughness R_(a) was 4.7microns. The workpiece was placed in a polypropylene straight-sidedthick-walled sealed cylindrical container with 84 mm internal height and60 mm internal diameter. Abrasive agglomerates (200 g, 720-micron cubescomprising P600 aluminum oxide FRPL grit and a vitrified binder from 3MCompany, Maplewood, Minn.) was placed in the container along with theworkpiece and 100 g of water. The LabRAM was run at 100% intensity inthe auto frequency mode for 30 mins. Afterward, the roughness R_(a) ofthe workpiece on the inside surface the tube was 2.9 microns and outsidesurface of the tube was 2.4 microns. The mass loss of the workpiece was0.010 g.

Example 5

This example demonstrates abrading additively manufactured polymer withloose abrasive grain.

The workpiece was an additively manufactured FormLabs Clear Resin(methacrylic acid esters with a photoinitiator) 20-mm diameter tube with2 mm walls. The workpiece was printed by stereolithography (initialroughness S_(a)=51 microns). The workpiece was placed in a polypropylenestraight-sided thick-walled sealed cylindrical container with 84 mminternal height and 60 mm internal diameter. P120 semi-friable fusedaluminum oxide BRFPL (100 g, Imerys) was placed in the container alongwith the workpiece. The LabRAM was run at 100% intensity in the autofrequency mode for 30 mins. Afterward, the roughness S_(a) of theworkpiece on the surface of the tube was 2 microns (98% improvement).The mass loss of the workpiece was 0.18 g (8% of the total initialmass).

Example 6

This example demonstrates peening of additively manufactured aluminumalloy (AlSi 10 Mg).

The workpiece was an additively manufactured aluminum alloy (AlSi 10 Mg)20 mm diameter tube with 2 mm thick walls. The part was printed by theDirect Metal Laser Sintering (DMLS) method (initial roughness R_(a)=4.7microns). The workpiece was placed in a polypropylene straight-sidedthick-walled sealed cylindrical container with 84 mm internal height and60 mm internal diameter. Spherical zirconia milling media (250 g, 3 mmdiameter, Retsch, Haan, Germany) was placed in the container along withthe workpiece and 50 g of water. The LabRAM was run at 100% intensity inthe auto frequency mode for 15 mins. Afterward, the roughness R_(a) ofthe workpiece (both inside and outside surfaces of the tube) was 1.2microns.

Example 7

This example demonstrates peening of additively-manufactured stainlesssteel workpiece.

The workpiece was an additively manufactured 17-4 PH stainless steelbracket printed by DMLS within 3 M. After printing, the workpiece wasleft unfinished, with an initial roughness R_(a) of 11.7 microns. Theworkpiece was then placed in a polypropylene sealed cylindricalcontainer with 55 mm internal height and 80 mm internal diameter.Stainless steel round shot (100 g, 2 mm diameter, CousinsUK) was placedin the container along with the workpiece and 50 g of water. The LabRAMwas run at 100% intensity in the auto frequency mode for 60 mins intotal. The roughness R_(a) of the workpiece after 15 mins of processingwas 2.9 microns. After 60 mins, the R_(a) was 1.0 microns.

Example 8

This example demonstrates peening of an additively-manufactured cobaltchromium alloy workpiece.

The workpiece was an additively manufactured cobalt chromium alloy(Co-Cr130) 20mm diameter tube with 2 mm walls printed by DMLS. Theroughness R_(a) after printing was 11.6 microns. The workpiece wasplaced in a polypropylene sealed cylindrical container with 55 mminternal height and 80 mm internal diameter. Tungsten carbide spheres(100 g, 1 mm diameter, Bearing Warehouse Ltd., Sheffield, UnitedKingdom) were placed in the container with the workpiece along with 50 gof water. The LabRAM was run at 100% intensity in the auto frequencymode for 15 mins. Afterward, the R_(a) was 3.0 microns.

Example 9

This example demonstrates peening of an additively-manufactured titaniumworkpiece.

The workpiece was an additively manufactured titanium alloy (Ti6Al4V)rectangular tab (10×30×1 mm) printed by DMLS. The initial roughnessafter printing, R_(a) was 6.9 microns. The workpiece was placed in apolypropylene sealed cylindrical container with 55 mm internal heightand 80 mm internal diameter. Tungsten carbide spheres (120 g, 3 mmdiameter, Bearing Warehouse Ltd.) were placed in the container with theworkpiece along with 50 g of water. The LabRAM was run at 100% intensityin the auto frequency mode for 60 mins total. After 15 mins, the R_(a)was 4.4 microns, and after 60 mins, the R_(a) was 2.2 microns.

Example 10

This example demonstrates peening of a machined aluminum alloy (Grade:BS EN 755 6082-T6) workpiece.

The workpiece was a machined aluminum alloy (Grade: BS EN 755 6082-T6)cuboid 15.9 mm×3.2 mm×50 mm. The workpiece was scratched by hand with aP36 grade coated abrasive to a roughness R_(a) of 7.5 microns. Theworkpiece was placed in a polypropylene straight sided thick walledcontainer (from United States Plastic Corp., Lima, Ohio) with 84 mminternal height and 60 mm internal diameter. Spherical ceramic tumblingmedia (250 g of 3 mm K-Polish Premium Ceramic Tumbling Media, KramerIndustries) was placed in the container along with the workpiece. TheLabRAM was run at 100% intensity in the auto frequency mode for 30 min.The roughness R_(a) of the workpiece after 30 mins of processing was 1.8microns. The maximum compressive residual stress in the surface of thematerial was −100 MPa before the process. After the 30 mins ofprocessing, the maximum compressive residual stress was −250 MPa. Thedepth of the compressive stress in the surface increased by 100 microns.Results are reported in Table 1, below.

TABLE 1 Residual Residual Residual Residual compressive compressivecompressive compressive stress in the stress in the stress in the stressin the Depth from X direction X direction Y direction Y directionsurface of before RAM after RAM before RAM after RAM substrate/processing/ processing/ processing/ processing/ mm MPa MPa MPa MPa 0.02−100 −220 −80 −250 0.04 −75 −250 −60 −245 0.06 −40 −240 −30 −210 0.08 10−190 −10 −150 0.1 20 −120 10 −90 0.15 50 −50 20 −25 0.2 30 0 10 0 0.2530 20 0 20 0.3 30 20 10 20

All cited references, patents, and patent applications in thisapplication are incorporated by reference in a consistent manner. In theevent of inconsistencies or contradictions between portions of theincorporated references and this application, the information in thisapplication shall control. The preceding description, given in order toenable one of ordinary skill in the art to practice the claimeddisclosure, is not to be construed as limiting the scope of thedisclosure, which is defined by the claims and all equivalents thereto.

1-12. (canceled)
 13. A method of modifying a surface of a workpiece, the method comprising: providing a system comprising a sealed mixing vessel having an interior chamber containing the workpiece and working bodies, wherein the workpiece is loose within the interior chamber; uniaxially vibrating the sealed mixing vessel at a frequency between 15 hertz and 1 kilohertz, and at a vibrational amplitude between about 0.2 cm and 3 cm such that the working bodies impact the surface of the workpiece.
 14. The method of claim 13, wherein the frequency is at or near a resonant vibrational frequency of the system.
 15. The method of claim 13, wherein the surface of the workpiece is metallic.
 16. The method of claim 13, wherein the surface of the workpiece is polymeric.
 17. The method of claim 13, wherein the working bodies comprise shot particles.
 18. The method of claim 13, wherein the working bodies comprise abrasive particles.
 19. The method of claim 13, wherein said uniaxially vibrating the sealed mixing vessel imparts an accelerating force to at least one of the workpiece and at least some of the working bodies of at least 40 grams-force (9.8 millinewtons).
 20. The method of claim 13, wherein at least some of the working bodies are accelerated to a velocity of at least than one meter per second.
 21. The method of claim 13, wherein at least some of the working bodies have a mass greater than 0.001 gram.
 22. The method of claim 13, wherein the interior chamber has a volume, and wherein a ratio of the total volume of the working bodies to the volume of the interior chamber is less than 0.8.
 23. The method of claim 13, wherein the interior chamber further contains a fluid.
 24. The method of claim 13, wherein the interior chamber further contains an etchant for the workpiece. 